The present invention relates to a liquid crystal composition and a liquid crystal shutter arrangement containing such a composition, for example a liquid crystal display device including a multiplicity of liquid crystal shutters.
Ferroelectric liquid crystal compositions suitable for fast, high-contrast displays with high levels of multiplexing and wide operating ranges are known (see for example J. C. Jones et al, Ferroelectrics, 1991, Vol. 121, pp91-102, and J. C. Jones et al, Displays, Vol. 14, No. 2, 1993, pp 86-93) which exhibit response time(xcfx84)-operating voltage (V) curves having a minimum (xcfx84-Vmin curves) when operated with pulsed switching drive schemes. Such compositions have significant dielectric biaxialities and relatively low values of the spontaneous polarisation Ps. The viscosity of these compositions increases with decreasing temperature. However, the spontaneous polarisation Ps of the liquid crystal molecules generally increases with decreasing temperature T (see FIG. 10), but the corresponding increase in speed of switching is insufficient to overcome the effect of increased viscosity. The result of this is that the xcfx84-Vmin curves of these compositions are temperature-dependent (see FIG. 11). Thus, the operating voltage signal required to switch the state of the liquid crystals changes with temperature. Accordingly, the required switching of the display may fail to take place at some temperatures encountered in use. In the case of global temperature changes, i.e. where the temperature of the whole display changes more or less uniformly, this can be at least partly compensated for varying the operating voltage signal depending upon the temperature of the display, or by controlling the temperature of the display. However, neither of these measures represents an ideal solution and does not easily permit temperature compensation where parts of the display are at different temperatures.
Additionally, problems also arise with analogue greyscale devices, where not only white and black are to be displayed, but also intermediate greys. In such devices, it is even more important than in digital white/black displays to reduce the temperature-dependence of the switching characteristics.
It is an object of the present invention to provide an improved liquid crystal composition which can enable the above problem to be obviated or mitigated.
According to a first aspect of the present invention, there is provided a liquid crystal composition comprising a host material capable of exhibiting a smectic C phase, and a chiral dopant material which imparts or enhances chirality in the host material, said composition exhibiting temperature-dependent response time-voltage curves with a voltage minimum (xcfx84-Vmin curves), wherein (a) the chiral dopant material has at least two chiral centres, (b) the chiral centres individually produce, in the liquid crystal composition, different and complementary temperature coefficients of spontaneous polarisation within a predetermined temperature range, and (c) the chiral centres are present in the liquid crystal composition in relative amounts such that regions of the xcfx84-Vmin curves across the predetermined temperature range are substantially coincident.
The chiral centres may be provided as chiral moieties in one or more chiral dopants.eg in a single chiral dopant compound or in different chiral dopant compounds.
The predetermined temperature range is typically a range of at least 20xc2x0 C., and preferably 20 to 40xc2x0 C.
The required variation of spontaneous polarisation with temperature in a liquid crystal composition according to the present invention is shown in FIG. 12. An inversion point (see the dotted line) may or may not be present. FIG. 13 illustrates a possible case where two dopants A and B separately produce different Ps curves (PsA and PsB, respectively), whilst the use of these two dopants together produces a Ps curve (Ps(A+B)) which is proportional to the sum of the squares of PsA and PsB, or which is proportional to PsA+PsB+2PsAPsB, ie a non-linear addition of Ps.
The result sought in a liquid crystal composition according to the present invention is substantial overlapping of the xcfx84-Vmin curves for substantially all temperatures within an operating temperature range (T1 to T2) of a device incorporating such a composition (see FIG. 14) so as to allow addressing of the device using the same voltage and addressing time at any temperature within such range.
The total amount of chiral dopant material may be up to about 25% based on the total weight of the liquid composition, and is more usually 2 to 10 wt %, and preferably is about 2 to 5%.
The first chiral centre may exhibit a spontaneous polarisation having an opposite sign to that of the second chiral centre.
In the case where the spontaneous polarisation of a liquid crystal composition containing the first and second chiral centres is too low for fast switching to occur, at least one further chiral centre having a relatively high temperature coefficient of spontaneous polarisation can be included so as to increase the spontaneous polarisation of the resultant liquid crystal composition and yet not have too great an effect on the overall temperature coefficient of spontaneous polarisation of the composition. An example of such a further chiral centre is that in a dopant has the following formula (I): 
Either chiral form of such dopant may be utilised (see dopants (VIII) and (IX) below).
The first chiral centre may be provided by a first chiral dopant having a lactate moiety in the chiral region. Examples of these are the compounds having the following formulae (II) and (III): 
Other examples of first chiral dopant are fluoroterphenyl compounds having a chiral centre in an end group of the molecule. An example of this is a compound of the following formula (IV): 
The above first dopants typically have a relatively low temperature coefficient (xcex3) of spontaneous polarisation.
With regard to the second chiral moiety, this preferably has a relatively high xcex3. The second chiral moiety may be provided by a second chiral dopant based on a Bahr-Heppke C-series type compound [for example, compound (V)] wherein a biphenyl compound has a branched chain alkylcarbonyloxy substituent group which is halo-substituted (typically chlorine) at the chiral centre. The biphenyl group in such dopant may be fluoro-substituted, preferably difluoro-substituted, on the phenyl ring which is closest to the chiral centre. Typical examples are those of the formulae (V) and (VI): 
The compound (VI) is believed to be novel. Thus, the present invention also relates to said compound (VI) per se.
In the case where the chiral dopant material comprises a compound of the formula (III) as the first chiral dopant and a compound of the formula (V) as the second chiral dopant, it is preferred for these dopants to be present in approximately equal proportions by weight.
In the case where the chiral dopant material comprises a dopant of the formula (III) and a dopant of the formula (VI), it is also preferred for these two dopants to be present in the liquid crystal composition in approximately equal quantities by weight.
The particular combinations of the first and second chiral centres (and the further chiral centre when present) required to produce the best effect are of course dependent upon the nature of the host material. The host material is normally a mixture of compounds capable of exhibiting phase changes upon cooling so that the composition passes from the isotropic phase (I) through chiral nematic (N*) and smectic A (SmA) phases before reaching the operative smectic C* phase. This is referred to as an I-N*-SmA-SmC* phase sequence.
As far as the host material is concerned, one type of host material is based on a mixture of diphenyl-carbonyloxyphenyl compounds. An example of this is a mixture (H1) consisting of equal amounts of the following such compounds: 
Another type of host material is based on a mixture (AS500) of the following dialkyldifluoroterphenyl compounds: 
A further type of host material is based on a mixture of fluorinated phenylpyrimidines which may also contain at least one fluorinated terphenyl compound for enhancing the formation of a certain phase or phases in the liquid crystal composition. Examples of this type of host material are described in copending British Patent Application No. 9719822.0 filed on Sep. 17, 1997, whose disclosure is incorporated herein by reference. Particularly preferred is the host material utilised in Example 10 described therein.
The liquid crystal composition may further include other additives which are per se known in the art, for example for producing improved phase transitions, birefringence, viscosity and cone angle, memory angle and layer tilt.
According to a second aspect the present invention, there is provided a liquid crystal shutter comprising a pair of mutually spaced substrates upon which are provided respective opposed alignment surfaces which are spaced apart, and a layer of a liquid crystal composition according to said first aspect of the present invention filling the space between the alignment surfaces.